This invention pertains to a charged-particle-beam projection-optical system that transfers a pattern from a mask onto a sensitive substrate (a wafer, etc.) using a charged-particle beam. More specifically, it pertains to a charged-particle-beam projection-optical system that can reduce the demand for stability and low noise in deflector power sources, while still enabling high-precision, high-throughput pattern transfer.
Direct-write electron-beam exposure systems provide high-precision, high-resolution image formation, but at low throughput rates. A variety of other electron-beam-based exposure systems have been developed with the goal of improving throughput. Currently, hybrid exposure systems (also referred to as cell-projection systems), character-projection systems, and block exposure systems have each been realized to some degree.
In a hybrid exposure system, small identical patterns (approximately 51 xcexcm square on the wafer) of a repetitive circuit portion are repeatedly transferred as individual units, using a mask on which multiple instances of the pattern have been formed. In such an exposure system, the non-repetitive portions of basic semiconductor integrated circuit devices (DRAMs, etc.) reduce potential throughput by about one order of magnitude because a long time is required to delineate the non-repetitive portions of a circuit. Furthermore, the hybrid exposure system has virtually no advantages in the production of microprocessors and any other circuits of a less repetitive nature, demand for which has been increasing.
Electron-beam reduction-projection devices have been proposed (e.g., see Japan Kokai Patent Publication No. HEI 5-160012). Such electron-beam projection-exposure systems are intended to provide vastly higher throughput than the hybrid exposure systems, while also being particularly useful for production of microprocessors.
Electron-beam projection-exposure systems irradiate an electron beam onto a mask defining the circuit pattern for one entire semiconductor chip, and reduce and project an image of the pattern in the irradiated area of the mask by means of a two-stage projection lens. It is generally not possible to provide an electron-optical field of sufficient size and quality to transfer the entire mask pattern at once. Accordingly, the field of the optical system may be divided into multiple sub-fields. The sub-field patterns may be sequentially transferred, and the electron-beam optical system may be altered or adjusted for each sub-field so as to maximize the performance of the imaging system for each sub-field individually. The entire circuit pattern is transferred by arraying the various sub-field images so that they are stitched together on the wafer. This is sometimes called a xe2x80x9cstitcherxe2x80x9d system. The basics of one such system are disclosed, for example, in U.S. Pat. No. 5,260,151.
Symmetric electromagnetic doublet lens systems are commonly known as a lens arrangement providing for good imaging performance with reduced aberrations. A symmetric electromagnetic doublet lens system is one in which (1) the lens structures (pole bore diameter, lens gap) of the two lenses that make up the two-stage projection lens are point-of-similarity symmetric about the entrance pupil (i.e., they are point-symmetric about the entrance pupil or crossover point, but with dimensions on the wafer side of the entrance pupil being reduced by the reduction ratio), (2) the magnetic field polarity of each lens is opposite that of the other lens, and (3) the ampere-turns of the excitation coils of both lenses are the same (see M. B. Heritage, J. Vac. Sci. Technol. 12(6):1135-1140 (November/December 1975)). With this arrangement and configuration all aberrations in the xcex8 direction, distortion, and transverse chromatic aberration are canceled.
Off-axis aberrations may be reduced in electron-beam exposure devices by using axial shift-type electromagnetic lenses, such as MOL and VAL, etc. For MOL (Moving Objective Lens), see H. Ohiwa, et al., Electron Commun. Jpn 54-B:44 (1971); for VAL (Variable Axis Lens), see H. C. Pfeiffer et al., Appl. Phys. Lett. 39(9.1):775-776 (November 1981).
Third-order geometric optical aberrations may be eliminated by the use of multiple deflectors in the projection-optical system. See, for example, T. Hosokawa, Optik 56(1):21-30 (1980). (The references mentioned above are hereby incorporated herein by reference.)
In electron-beam projection systems utilizing magnetic deflectors for aberration correction, it has generally been necessary to provide a high-precision power source, with output control accuracy of 10xe2x88x926 or greater, for the coil of each deflector. Any drift or ripple in the deflector power source causes the position of the projected image to drift or fluctuate, which causes deterioration in the stitching precision and in the line-width precision in the transferred images.
A charged-particle-beam projection-reduction optical system according to the present invention includes aberration-correcting deflectors positioned between a mask position and a substrate position, with at least two of the deflectors being arranged to be excited by a single power source.
The at least two deflectors are positioned and structured such that any fluctuations in power supplied by the power source tend to cause offsetting effects in the at least two deflectors, such that any such fluctuations will have little or no effect on the position of the image on the substrate. The at least two deflectors may consist of a pair of deflectors arranged to be point-of-similarity symmetric about a crossover point, with an equal number of turns in the coils of each of the pair of deflectors. Multiple such pairs, each powered by a single power source, may be employed.
One or more of the deflectors, desirably on the mask side of the crossover point, may include a main coil and a sub-coil, with the main coil being powered by a power source shared with one or more other deflectors, and the sub-coil being powered by another power source.
Grouping (or pairing) and balancing the deflectors as taught herein relaxes the requirements on deflector power supplies, thereby reducing the negative effects in the imaging system of deflector-power-supply ripple and drift.
The imaging system is desirably structured so as to allow optimum aberration correction with deflector power being adjustable only in pairs or groups. Alternatively (or in addition), provision of a small sub-coil on one or more of the deflectors preserves some of the flexibility of non-balanced deflectors while still reducing negative effects of power supply variations. In either case, high- or higher-performance imaging can be achieved at less cost.